The present invention relates to an apparatus for measuring the basis weight, and more particularly, to an apparatus for measuring the basis weight of a web-shaped material under test.
U.S. Pat. No. 3,681,595 describes an arrangement of a radioactive radiation source on one side of a material under test and a radiation detector responsive to the radioactive radiation on the opposite side of the material under test for the on-line measurement of the basis weight of a web-shaped material under test. The material under test described therein is paper. As radioactive radiation sources, a .beta. emitter (e.g., Promethium 147, Krypton 85 and Strontium 90) may be used. The radiation detector may consist of an ionization chamber or of a semiconductor detector, respectively. By measuring the transmission of the electrons emitted by the radioactive .beta. radiation source, the basis weight of the material under test may be evaluated. In order to achieve a satisfactory signal-to-noise ratio, the radioactive radiation source must show a certain activity. On the other hand there is a desire to use a source having an activity as small as possible because of the radioactive radiation.
Also, from German patent DE-A-2800925, the measurement of the basis weight can be achieved by arranging a .beta. radiation source and a .beta. radiation detector side by side on the same side of the material under test and to take care by the provision of a magnetic field which extends in the plane of the material under test that the .beta. particles emitted from the radiation source reach the radiation detector on curved trajectories. Thus, the .beta. particles pass twice through the material under test.
The present invention improves the signal-to-noise ratio at a predetermined activity level of the radiation source. The present invention has in view a focusing of the electrons emitted by the .beta. radiation source due to a magnetic field and due to the Lorentz force exerted by this magnetic field onto the electrons. By focusing of the emitted electrons within the lower half space in a direction versus the material under test, more .beta. particles are available for the measuring of the transmission without changing the activity of the radiation source.